The International Maritime Organization (IMO) number is a unique reference for ships, registered ship owners and management companies. IMO numbers were introduced to improve maritime safety and security and to reduce maritime fraud. They consist of the three letters "IMO" followed by unique seven-digit numbers, assigned under the International Convention for the Safety of Life at Sea (SOLAS).
In 1987 the IMO adopted resolution A.600(15), aimed at the "enhancement of maritime safety and pollution prevention and the prevention of maritime fraud" by assigning to each ship a permanent identification number. The IMO number remains linked to the hull for its lifetime, regardless of changes of names, flags, or owners. The IMO adopted the existing unique 7-digit numbers applied to ships by Lloyd's Register since 1969, which were modified from 6-digit numbers introduced in 1963.
SOLAS regulation XI/3, which was adopted in 1994 and came into force on 1 January 1996, made IMO numbers mandatory. It was applied to cargo vessels that are at least 300 gross tons (gt) and passenger vessels of at least 100 gt.
In December 2002, the Diplomatic Conference on Maritime Security adopted a number of measures aimed at enhancing security of ships and port facilities. This included a modification to SOLAS Regulation XI-1/3 to require ships' identification numbers to be permanently marked in a visible place either on the ship's hull or superstructure as well as internally and on the ship's certificates. Passenger ships should also carry the marking on a horizontal surface visible from the air.
In May 2005, IMO adopted a new SOLAS regulation XI-1/3-1 on the mandatory company and registered owner identification number scheme, with entry into force on 1 January 2009.
The regulation provides that every ship owner and management company shall have a unique identification number. Other amendments require these numbers to be added to the relevant certificates and documents in the International Safety Management Code (ISM) and the International Ship and Port Facility Security Code (ISPS). Like the IMO ship identification number, the company identification number is a seven-digit number with the prefix IMO. For example, for the ship Atlantic Star (IMO number: 8024026), IMO 5304986 referred to the former ship manager Pullmantur Cruises Ship Management Ltd and IMO 5364264 to her former owner, Pullmantur Cruises Empress Ltd.
IMO identification numbers for ships, companies and registered owners are assigned by IHS Markit (previously Lloyd's Register-Fairplay). For new vessels, the IMO number is assigned to a hull during construction, generally upon keel laying. Many vessels which fall outside the mandatory requirements of SOLAS have numbers allocated by Lloyd's Register or IHS Markit in the same numerical series, including fishing vessels and commercial yachts.
An IMO number is made of the three letters "IMO" followed by a seven-digit number. This consists of a six-digit sequential unique number followed by a check digit. The integrity of an IMO number can be verified using its check digit. This is done by multiplying each of the first six digits by a factor of 2 to 7 corresponding to their position from right to left. The rightmost digit of this sum is the check digit. For example, for IMO 9074729: (9×7) + (0×6) + (7×5) + (4×4) + (7×3) + (2×2) = 139.